1. Field of the Invention
The present invention relates to a sheet processing apparatus having a conveying unit for performing a post process such as aligning or binding sheets on a processing tray and discharging an obtained sheet bundle to a paper receiving tray.
2. Description of the Related Art
In U.S. Pat. No. 5,385,340, a sheet post-processing apparatus is described. The apparatus stacks and stores a plurality of sheets with images recorded, which are sent from an image forming apparatus, on a processing tray which is a fixed loading section, staples the rear ends of the sheets, and forms a sheet bundle. Thereafter, the stopper plate strikes the sheet bundle and furthermore, the stopper plate moves and presses out the sheet bundle from the processing tray onto the storing tray. In this apparatus, the stopper plate is positioned firstly at the rear end of the processing tray and aligns the rear ends of sheets sent from the image forming apparatus. When sheets of the selection number are stacked on the processing tray, the stapling unit operates and staples the rear ends of sheets, and the stopper plate starts movement and conveys the sheet bundle.
According to the art described in U.S. Pat. No. 5,385,340, the stopper plate is opposite to almost overall the sheet bundle in the width direction and the sheet bundle moves to the stopper plate, so that the stopper plate itself is made larger, and drive force is required, thus the drive unit is made larger inevitably. Ideally, a belt is stretched between a pair of pulleys, and an arm is installed at a part of the belt and is hooked almost at the central part of a sheet bundle, and the other part of the sheet bundle is just loaded on a roller. When the belt travels, and the arm moves, and the roller is driven to rotate, thus the sheet bundle can be conveyed, the drive force can be reduced and the miniaturization of the drive unit can be promoted. However, even if such a constitution is adopted, unless the moving speed of the arm against the rotational speed of the roller is set under good conditions, the conveyance of the sheet bundle becomes unreasonable and the conveyance reliability is lost.
Generally, the moving speed of the arm moving along the straight portion of the belt is in synchronization with the rotational speed of the roller, so that the movement of the sheet bundle along the sheet placing surface of the processing tray provides no trouble. However, when the arm moves in the peripheral direction of the pulley at the end on the downstream side in the sending direction and reaches a predetermined angle, the sheet bundle can be discharged from the processing tray onto the paper receiving tray, though the speed in the radial direction is increased and in this state, the peripheral speed becomes faster than the moving speed on the straight portion. On the other hand, the rotational speed of the roller is fixed always and the conveyance of the arm and conveyance of the roller are shifted from each other. Therefore, when the arm moves in the peripheral direction of the pulley, the speed against the sheet bundle is increased and the arm cannot synchronize with the roller. After all, the roller breaks, while the arm forcibly presses the sheet bundle and a problem arises that the paper receiving alignment of the sheet bundle lowers.